Referring to FIG. 1, a conventional uninterruptible power supply (UPS) system 1 includes an AC/DC power supply 11, and a plurality of direct-current (DC) UPS devices (e.g., the DC UPS devices 121 and 122) electrically connected to the AC/DC power supply 11 and a load 100. The AC/DC power supply 11 is configured to receive mains electricity, to convert the mains electricity (i.e., alternating-current (AC) electric power) into DC electric power, and to output the DC electric power to the load 100 via a supply bus 13. When the mains electricity is normal, the DC UPS devices 121 and 122 are charged by the DC electric power from the AC/DC power supply 11. When the mains electricity is out of order, for example, power blackout, the DC UPS devices 121 and 122 are configured to supply the load 100 with backup voltages via the supply bus 13 to ensure normal operation of the load 100 (e.g., a server system). In a case that the conventional UPS system 1 is a high-capacity UPS system, the DC UPS devices 121 and 122 are electrically connected in parallel and operate in redundant distribution mode for increasing the reliability and shortening the maintenance time. After operation of the mains electricity has been recovered, the DC UPS devices 121 and 122 stop supplying the backup voltages and are electrically disconnected from the supply bus 13, so that the AC/DC power supply 11 provides the DC electric power to the load 100.
However, voltage perturbation may occur on the supply bus 13 the instant the DC UPS devices 121 and 122 stop supplying the backup voltages. Further, referring to FIG. 2, the DC UPS devices 121 and 122 may not stop supplying the backup voltages at the desirable time, worsening the voltage perturbation on the supply bus 13 and increasing instantaneous load of the DC UPS devices 121 and 122.